The prognosis of patients with coronary heart disease (CHD) is closely related to the degree of coronary luminal obstruction. In population studies, the incidence and severity of coronary atherosclerosis on arteriography is correlated with diet and plasma lipoprotein cholesterol. The proposed investigation will test directly by sequential coronary arteriography whether an intensive hypolipidemic program of diet with and without drugs can reverse or retard the progression of obstructive coronary artery disease in humans. The effect of therapy will be studied in 90 normolipidemic patients who will have received medical treatment for their coronary disease and in 90 patients who will have received saphenous vein coronary artery bypass grafts. Therefore, the effect of therapy will be determined in native coronary arteries that are in a native coronary circulation and that are in a circulation that has received aortocoronary bypass grafts. The potential benefit of treatment on stenosis in the bypass grafts also will be studied. Patients will be eligible for the study who have significance stenoses in one or more major coronary arteries on angiography, and who have plasma cholesterol levels below 250 mg/dl. Patients will be randomized into three groups: 1) standard dietary advice only, 2) an intensive dietary program, 3) the intensive diet plus hypolipidemic drugs as needed to lower plasma total cholesterol to 160mg/dl or LDL-cholesterol to 105mg/dl. After 24 months of therapy, a second coronary arteriogram will be performed. Computerized densitometric analysis of the pre- and post-treatment angiograms will quantitate the change over two years of therapy in minimum diameter of stenosed coronary arterial segments. The technique will also quantitate the change in mean diameter of coronary arteries that are visually normal. The precision of the technique will enable the study to detect a mean change of less than 10% in arterial luminal size.